This relates to JENKINS-7915. I can workaround the problem in clearcase plugin, but I believe this should be fixed in core.
Thing is, AbstractBuild#getEnvironment relies on Run#getEnvironment which retrieves the computer the build occurs using

Computer.currentComputer()

However, it only works if the current thread is an executor, that is, only if the build is running. In AbstractBuild, using getBuiltOn().toComputer(), we can actually retrieve a valid reference for the computer the build executed even if it isn't running anymore.